1. Field of the Invention
The present invention relates generally to gas sensing devices. More specifically, the present invention relates to a gas sensing device for measuring oxygen levels in fuel tanks.
2. Description of Related Art
Since 1959, a number of aircraft fuel tanks have unexpectedly exploded. Typically, the explosions occurred when an unknown ignition source ignited the fuel/vapor mixture in the fuel tank. Fuel/vapor mixtures are created during consumption of fuel within the fuel tank by engines of the aircraft. The consumed fuel leaves a space within the tank which generally fills with atmospheric air containing oxygen. The presence of both a flammable gas and the fuel/vapor mixture within the space creates the potential for an explosion within the fuel tank upon ignition. The industry has responded with various methods and apparatuses, as discussed in Air Safety Week, Vol. 15 No. 16, Apr. 16, 2001, “Fatal Explosion Highlights Hazard of Flammable Vapors in Fuel Tanks.”
One prior art method which reduces fuel/vapor combustion includes the elimination of combustible gases from the fuel tank. This prior art method fills space within a fuel tank with an inert gas. The presence of the inert gas within the fuel tank deprives the fuel/vapor mixture of a flammable gas necessary for combustion. Nonetheless, the need to continuously fill the fuel tank with an inert gas and the attendant high costs associated therewith do not make this an attractive alternative for aircraft manufacturers.
A more efficient method in accordance with the prior art includes flooding the tank with inert gas when oxygen levels become high. This method requires continually measuring oxygen levels in a fuel tank. The sensors must stay at a constant temperature level in order to accurately measure oxygen levels. However, temperatures of fuel tanks in vehicles tend to fluctuate depending on the outside temperature. Therefore, the oxygen sensor's temperature must be kept at a constant level in order to allow accurate measurements by the oxygen sensor.
Prior art attempts to keep the temperature of a gas sensor constant include heating the gas sensor with electric resistance heaters when the temperature is low. However, these methods are not suitable for use in fuel tanks, as electrical current applied to the electrical resistance heaters may potentially ignite the fuel/vapor mixture within the tank, again making this an unattractive option for aircraft manufacturers.
Therefore, a need exists for a method and apparatus which maintains a temperature of a gas sensor at a constant level. This new method and apparatus should minimize the introduction of elements which may ignite a fuel/vapor or other hazardous mixture within a fuel tank and/or space.